Amino acids currently have application as additives to animal feed, nutritional supplements for human food, components in infusion solutions, and synthetic intermediates for the manufacture of pharmaceuticals and agricultural chemicals. L-glutamic acid is used as a flavor enhancer for food with a world market of over 1 billion dollars annually. L-lysine and methionine are large volume additives to animal feed, and L-tryptophan and L-threonine have similar potential applications. L-phenylalanine and L-aspartic acid have very important markets as key components in the manufacture of the sweetener aspartame. Infusion solutions require a range of amino acids including those essential in human diets.
Methods developed for the synethesis of amino acids involve fermentation, chemical synthesis, extraction from protein hydrolyzates, and enzymatic bioconversions. Chemical synthetic methods generally involve the initial formaation of a racemic mixture, followed by the resolution of this mixture to yield the optically active product. The resolution may be accomplished either chemically, by fractional crystallization of diasteromeric salts of the amino acid, or if desired, enzymatically using the enzyme L-aminocylase. The undesired isomer can be re-racemixed and then recycled through the process. Fermentation methods suffer from problems of slow rates of conversion, costly purifications, and very high capital investments. Extraction from protein hydrolyzates is used only in a few cases because the amino acid of interest is a relatively low percentage of the total protein. Enzymatic conversions offer advantages primarily due to reduced capital investments, lower purification costs, and higher rates of conversion.
One such previously described process involves the transamination of a given 2-ketoacid to the corresponding L-amino acid (U.S. Pat. No. 4,518,692 (May 1985)). In that process, L-aspartic acid and a 2-ketoacid are reacted in the presence of a transaminase to form the desired L-amino acid and oxaloacetate, followed by decarboxylation of said oxaloacetate to form pyruvate. The essentially irreversible decarboxylation of oxaloacetate drives the entire process to completion to form L-amino acids in yields approaching 100% of theoretical from the corresponding 2-ketoacids. The reaction is summarized in Scheme 1. ##STR1##
This process has the requirement that the transaminase employed in the practice of the invention accept L-aspartic acid as the amino group donor. There exist certain transaminases with desirable characteristics and specificities for use in biocatalytic processes, but which cannot use L-aspartic acid as the amino donor. The present invention is an improvement on the process of U.S. Pat. No. 4,518,692 described above; the present invention provides a method for achieving the advantages of using L-aspartic acid, as well as any of a variety of other amino acids, as the amino group donor in transamination reactions involving enzymes that utilize the amino acid so employed either poorly or not at all as an amino group donor.